This invention concerns a method of reducing imperfections in blown plastic film. More particularly, it concerns a method of reducing "breakdown" in blown polyethylene film.
Blown film is typically formed from low-density polyethylene. In the blown film process, polyethylene pellets are melted and extruded from an annular die to form the film envelope. In the extruder and die system, there are typically "dead spots" or "hangup areas" where the molten polyethylene does not move rapidly, resulting in long heat histories for portions of the melt. It is theorized that the polyethylene in these areas tends to be decomposed or cross-linked, forming particles that are of a distinctly different viscosity from the rest of the molten polymer. As portions of the polymer are flushed from these hangup areas, they cause imperfections in the film, known as "stripes", "memory lines", "arrowheads" or "chevrons", and gels, collectively known as "breakdown", which often disrupts production in the form of "breakoff" or "tearoff". These imperfections cause a lowering of the quality of the blown film and are economically undesirable. It is recognized in the blown film industry that blown film dies that operate at low pressures usually experience more "breakdown" than do high pressure dies. Since the resin producer does not known in what machines his resin will be used, it is prudent to furnish a resin having a reduced tendency for "breakdown". The incidence of "breakdown" is typically (or can also be) reduced by lowering the extruder output and thus the melt temperature of the polymer, reducing the tendency toward decomposition of the polymer. Therefore, any device or method for decreasing "breakdown" without decreasing output is a desirable advance in the art.